There is a legal requirement that vent valves on fuel tanks on highway trucks be closable automatically, so that if a truck is tilted or tipped over, there will be no spillage of fuel through the vent valve. It is also necessary that the vent valve be open during normal operation to allow air into the tank as the fuel therein is decreased, and to allow gas to flow out of the tank if the fuel is being increased in the tank. The requirements are the same when the tank and/or fuel therein contracts or expands. It is also necessary that there be safety releases against excessive pressure and excessive temperature in a fuel tank.
Government regulations require that all fuel tank vent valves be designed to prevent liquid spillage at any attitude of the fuel tank.
In the best prior art known to applicant, a ball valve, although not bouyant, is seated too easily, under the influence of gas flowing out of the tank. This occurred because the ball was too close to the seat and the gas pressure activity on the wall around the ball and on the ball was sufficient to seat the valve when it should have remained open. In addition the non-bouyant ball valve would not always seat when vertical or when at less than 30.degree. angles of tilt in the presence of liquid fuel. This was due to respective inaction of two other ball elements in the valve.